How does Elon Musk’s SF-to-LA-in-30-minutes Hyperloop work?

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In July of last year, Tesla and SpaceX founder Elon Musk let slip that he was working on the Hyperloop — an ultra-fast mode of transportation that will get you from downtown Los Angeles to downtown San Francisco is under 30 minutes. This is a distance of roughly 340 miles, and would require speeds of around 700 mph, or almost the speed of sound. Perhaps most importantly, though, Musk said the Hyperloop will only cost around $6 billion — compared to the $60 billion of the proposed high-speed rail link connecting the two cities. So far, so good, except for one niggling issue: Musk still hasn’t told us how he intends to build it.

Some of the world’s brightest minds have speculated that a vacuum tube is the only way to do it — but before that idea could even get off the ground, Musk said that the Hyperloop is not based on an evacuated tunnel. With that possibility ruled out, there aren’t actually that many ways of safely and economically propelling carriages at 700 mph (1126 kph). Furthermore, when you factor in Musk’s comments that the Hyperloop “can never crash,” has no need for rails, and is “immune to weather,” the architecture of the system becomes a real head-scratcher. Oh, did I mention that Musk envisions the entire system being self-powered by solar panels, and that it somehow stores energy inside the system itself, without the need for batteries?

How, then, might the Hyperloop work?

Acoustic levitation

At that speed, the biggest enemy is always going to be air resistance, which is why a vacuum tunnel is usually the favored solution: In a vacuum there is no air resistance (drag), and thus you can essentially move as fast as you like — much like a spacecraft barreling through the great black expanse. But it isn’t an evacuated tube, so it must be something else. Not to mention, a vacuum tunnel would definitely not fulfill the “can never crash” factor; poke a hole in a vacuum tube, and the results would be very messy indeed.

What we need is another way of efficiently reducing drag. Just recently, we wrote about a research group that levitated arbitrarily shaped objects in acoustic waves. This technique involves an acoustic phenomenon called standing waves — essentially, waves that are held in place by interference. If you imbue these waves with enough power (volume) and hit just the right frequency, you can levitate an object. Standing waves, as the name implies, don’t move — but Björn Smedman and Charles Alexander both theorize that, if you pump these waves into a loop (which we assume the Hyperloop is), and change up the acoustic parameters slightly, then it might be possible to carry vehicles on the edge of these waves as they travel around the loop.

It turns out that, by hitching a ride on the peak of a sound wave, you only really have to deal with drag caused by air density (linear), which is much less than drag caused by air velocity (square).If you pump enough power into the acoustic wave (i.e. increase the amplitude), the air density increases but the relative air velocity drops. In effect, the vehicle in the wave is stationary, in reference to its surroundings. Eventually, as the sound wave gets stronger and stronger, you achieve almost adiabatic travel — travel that loses no energy at all to the environment via drag or friction.

Inside the Hyperloop. [Image credit: Charles Alexander]

In theory, this process is so efficient that solar panels on top of the loop (a very large surface area!) can power the system. The acoustic waves, traveling continuously around the loop, would effectively act as energy storage.

Embarkation and disembarkation

While acoustic waves neatly solve the traveling-at-almost-the-speed-of-sound bit, they don’t explain how you would embark and disembark from the Hyperloop. The best guess at the moment is that there will be an extra section at each end of the loop for managing acceleration and deceleration. To board the Hyperloop, you will hop into a carriage at the San Francisco or Los Angeles terminus, and then be accelerated up to speed using a railgun before entering the main loop. At the other end, you will be gently decelerated before disembarking. This neatly ties in with Musk’s comments that the Hyperloop will be a “cross between a Concorde and a railgun and an air hockey table.”

A proposed Hyperloop design, with a main section, plus acceleration and deceleration regions for embarkation and disembarkation [Image credit: John Gardi]

Finally, we should note that there are other ways of constructing a Hyperloop. It might turn out that Musk was just trying to be coy, and that it will actually be based on some kind of evacuated tunnel design. Instead of a vacuum, the tunnel could be flooded with a lighter (less-dense) gas such as helium, which produces less drag. Because there’s still a pesky force called gravity to overcome on Earth, he would still need a way of defeating rolling resistance (friction between the vehicle and the edges of the tunnel). Magnetic levitation (maglev), which has been extensively tested in Japan, might soon be ready for commercial use and is the obvious solution.

It also isn’t clear how any of the above approaches “can never crash.” In theory, if you poke a hole in a Hyperloop full of acoustic waves, the waves will very quickly dissipate and the vehicle will come to a standstill. This will probably happen very quickly, and probably quite painfully for the occupants — especially if you’re going around a corner at the time.

Still, with Elon Musk set to reveal his “Hyperloop alpha” plans on August 12, we’ll finally be able to stop theorizing about possible designs, and instead shift our focus to whether the plan actually stands up to Musk’s extraordinary claims.

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Nima

This is an intriguing idea. At first I must admit I was almost sure Elon’s idea was an evacuated tube maglev. But with more and more people sighting that this may not be the case, Im willing to consider other ways of doing this. While “riding” a standing linear wave seems to solve the drag problem between the vehicle and the surrounding air, I fail to see how it solves the drag problem between the stationary tube and the standing wave. The way I see it, fluid, traveling within a pipe, whether it’s a longitudinal wave form or a simple directional flow, still undergoes pipe flow loses. And these are very significant when you’re talking about a 350mile tube! And so while the vehicle to air resistance is only proportional to V, the air to tube resistance is also proportional to V, making the total come back to V squared, and you’re back to “square one” hehehe sorry couldn’t resist that one.
Anyway Id like who ever proposed this theory, to try and explain this little conundrum that I see.

Rangi J

Here’s my thoughts.

Perhaps the key is not to evacuate the tube but to alter the air density profile from being uniform over the cross-section to a state where the air density is much higher toward the edge of the “tube” than it is in the centre. This would be achieved by inducing a centrifugal force on the air within the Tube.

The craft would then travel in a more central area of the tube which whilst not a vacuum, would provide a much lower overall drag coefficient.

To achieve this aim i would construct the tube as follows. It would have an outer, hermetically sealed shell and then an inner sheath which would have slits at regular intervals. In the space between the inner and outer shell I would accelerate the air to a high speed. This would effectively ‘suck’ the air out of the inner sheath, creating a high pressure region between the inner sheath and outer shell and a lower pressure region (lower density region) inside the inner sheath. Eventually a state of relative equilibrium would be achieved where there would be effectively no air transfer from the inner sheath to the high speed regionThe craft would travel within the inner sheath.

I don’t have the software (or time) to produce an accurate model of how this would work, but I think that this would create a tube system that is certainly more conducive to high speed tunnel travel.

For me this would also tie in with Musk’s comment that it would be “like an air hockey table”

Prof. Peabody

Futurist nonsense. Intriguing in the same way a science special is on TV but this will not be built in anyone’s lifetime.

A evacuated tube transport system with linear motors is more efficient, safer, and would reach faster speeds than this also. The authors note about the piercing of such a tube being deadly is simply not true. The only problem with the tube is that the system itself is being proposed for dead centre of one of the worst earthquake zones on the planet.

But that makes me wonder what the “acoustical wave” would do to the tectonic plates in this area also.

sdf

What an idiot.

monkeyrun

While these interstate or intercity transportation system is nice. What we need is a public transportation system that works within cities. 30 mins from SF to LA? I would kill to be able to get from east LA to west LA in 30 mins.

Rangi J

Yes but it’s Musk’s money and he can do what he likes with it. There’s no doubt there would be a market for a 30 min SF to LA transportation system if it was realized and affordable.

Dozerman

Could it be a closed loop blower system similar to a lazy river at a themepark with some kind of low drag interior?

http://www.mrseb.co.uk/ Sebastian Anthony

Yeah, one of the other theories is a closed-loop wind tunnel type thing. It’s possible.

Dozerman

Hasn’t that already been done? I think it was New York where they had closed loop subways tunnels powered by fans.

CPPCrispy

This is what came to mind when I started reading the article.

John Gardi

Dozerman:

You don’t need a low friction surface, you need a laminar surface. Air makes a pretty good laminar surface, so instead of making the tube wall low friction, make the tube wall create a laminar surface in the air above it. Perfect opportunity because the air is already going 600mph.

You could even change the surface texture at the top and bottom of the tube to make up for gravity. (Thicker laminar surface at the bottom)

That’s old tech so it’s a probable choice. Just counter-intuitive, is all, because who’d think of doing this sort of stuff inside out, so to speak?

You’ll see, it’s simpler than you think.

tinker

Johnny Le

Since the air is going 600 mph, if there was a fire in the pod and the riders had to get out, or if a baby accidentally crawled into the tub, there would be no survivors, would there?

How about a acoustic wave/maglev hybrid. Maybe they could use maglev technology as a secondary system for safety. For instance, if the acoustic wave failed, the maglev system would allow the vehicle to gradually slow down by not allowing the vehicle to contact the sides, bottom, or top of the tunnel. This maglev system could also be a breaking system, in that if there is a vehicle ahead that is slower or is stopped, the maglev system could slow down and stop the other vehicle from crashing into the one in front.

John Gardi

CPP:

You don’t back up a complex system with an equally complex system. ANY backup must be simpler then the primary. The simpler the backup, the safer it is.

Sorry, I’ve done fail-safe and backup systems and the best backup is a primary system that fails gracefully! The fact that acoustic-wave came up at all shows that folk weren’t using their noggins and/or do proper research!

Repeat, I would never put folks on an acoustic-wave vehicle period that has a few milliseconds primary system failure flaw.

Not gonna happen!

tinker

convolution

The answer is obvious. Obviously he’s going to create a wormhole. The wormhole takes 29 minutes to create, and it will take occupants 1 minute to safely cross the wormhole to the other side.

some_guy_said

Good article.

Chris Muss

Musk said that the design was a cross between a rail gun, a Concorde,
and a air hockey table. This should make it obvious that the levitation
is provided by a cushion of air blown through many tiny holes, probably
in the outer shell of the carriage rather than in the walls of the tube.
This would be the air hockey table component. Then the acceleration
would be provided by the rail gun component (two parallel conductors
with current traveling in opposite directions, connected by a sliding
armature, which is connected to the carriage). The Concorde component
would seem to suggest that the carriage would be extremely aerodynamic. I
would also suggest that the design may involve channeling air from in
front of the carriage into the levitation system (air hockey table), and
ultimately out behind the carriage. The tube would be a simple steel
tube elevated on stilts. That’s my take.

John Gardi

Chris:

I’m with you on the passive tube, active pod debate (hell, there is no debate). Only way to be able to afford it. Rail is ‘passive’ in the same way.

tinker

Chris Muss

Yes, I agree, making the tube into an air hockey table surface would make the system very expensive, and prone to failure. If a single carriage fails it can be removed from service and the overall system can continue to function, but if the tube fails the whole system would have to be shut down for repair.

John Gardi

Chris:

So… that means the pods are active. I made a model with some tubing, radio control guts and a bike pump. The two props from the RC plane faced into each end of smaller tube, my ‘Pod’. So, I had higher air pressure in my li’le tube. Trial and error dremeling holes in the small tube gave me maximum ‘lift’. At full throttle, my pod floated quite freely. If I changed the speed of one prop it would move back and forth under control.

So far, so good. Active cars work. I don’t think (or care) whether Elon Musk has propulsion planned for his Pods… but it worked for me! ;)

Now the bike pump! I took the valve off the end so I could get the maximum air blast and duct taped the hose on one end of my ‘transport tube’. Fired up my pod and ‘old cannon style’ slid it in and let it float to the pump end. Pushed down on the plunger and…

Shot out of there like a bullet! Took me half an hour to put it back together enough so I could make sure it wasn’t a fluke.

It wasn’t. Again, not trying to second guess Elon and all, but he’s on to something, to be sure.

I’ve stopped worrying about the air in the Hyperloop system. It’s no big deal. They have to have about four time the mass of Pods to air for the curve to drop off enough, but that’s only 1000 pods at 2t each, 500 a ‘lane’, a little less than a mile apart.

BTW: That means there’s 500 tons of air in the entire Hyperloop system. Not bad for an 800 mile loop!

Anyway, that’s what I’ve worked out so far…

…and I haven’t even come close to stepping on Elon’s toes.

I can see I’ll have to do the original work myself though.

tinker

Chris Muss

Wow, it sounds like you’re very intent in figuring this thing out. Kudos to you. Nothing beats real world modeling.
I’d like to propose an additional idea if I may. Imagine that the pods can be accelerated by the railgun principle, with 2 parallel conductors, one on either side of the tube. The pod would act like a piston, pushing the air ahead of it and creating a low pressure area behind it. Each pod would be doing the same thing. With 1000 pods all pushing & “pulling” the air in the tube system, there wouldn’t be too much need for fans to move the air in the tube system. However, fans would be needed on the pod, as you described, to maintain the air hockey levitation effect.

Energy for these on-board fans could come from the parallel conductors in the tube walls. With only 2 pods stopped at any given time, and 998 in motion, there shouldn’t be too much concern about the stopped pods stalling the air flow.

Thucydides_of_Athens

Speculation on the Hyperloop is fun. I have seen ideas on the Internet that only Wile E. Coyote could love.

An interesting question is can the Hyperloop be scaled to carry a standard 20′ ISO shipping container? A high speed, high throughput point to point freight system might actually be far more useful, even if it can only move at 1/2 the passenger speed a 500kph system is 5X faster than truck or freight train traffic.

John Gardi

Thucydides:

Interesting that you should mention that. Even if the ‘form factor’ of an HL Pod is too small for a shipping container, anything that would fit in the back of a Fed-Ex truck would fit in a pod for sure. Lots of small stuff could need priority inter-city shipping, so cargo pods are a distinct possibility.

Cheers, good too see (even if it’s so very few) use their noggins on this one!

tinker

mlhoheisel

It doesn’t address the “air hockey table” or can’t crash. It is unlike the one guess Musk said was closest. The John Gardi suggestion illustrated that Musk gave the nod to suggests a system that moves the air column along with the transport pods as an alternative to a vacuum. That implies a means of reducing the friction of the air column moving at 700 mph against the tube walls. It’s not existing tech but a double walked tube that creates an air cushion something like an air hockey table curled up and stretched 400 miles might provide a low friction surface for both the transport pods and the air column that moves with them. In such a system the air column between the pods would make it effectively impossible for them to crash into each other acting like an air shock absorber…I think that’s the reference to “can’t crash”, it’s not a suggestion that the system can’t fail if the tube is violently ruptured, just that in normal operation the transport pods cannot crash into each other (unlike a vacuum tube transport).

John Gardi

mlhoheisel:

Go to the head of the critical thinking class! I had my drawing out for a month and folks were still debating how my one spelling mistake effected my credibility instead of doing critical thinking themselves!

I wrote about this traveling system a while back. It’s so weird that I saw it on a dream 3 years ago and wrote about it in my book. 12-21-12 The Revelation 12-12-21

From Kansas City to all corners of US in minutes.

John Gardi

Folks:

Yes, I’m that tinker! Thanks for doing some original work in this article. I literally cried when Popular Mechanics used my flow chart in such a poorly written article. I posted that image so people would think! Thank you for discussing the inplications of my image based on the fact Elon said it was close. Instead of parroting like everyone else, you included it in context. +1!

Now… about the acoustic levitation…

First, that’s young technology and I can tell Elon Musk has been thinking of this for a long time. We wouldn’t have even heard about it if he didn’t think it would fly (speaking of flying… he also has mentioned an electric VTVL supersonic jet, so…)

Also, Musk is pretty old school (for his age) when it comes to physics too. So don’t expect Hyperloop to contain any exotic technology. Just my guess.

I did however, use those last to points as ‘clues’ when I made the drawing. Maybe that’s why it was a ‘best guess’ and not the other clues.

Having said that, I looked into the tech you mention above and I wouldn’t dare send anything other then cargo in any system that uses it. Maybe that’s it’s place, who knows.

I’ve designed fail-safe systems before. One rule for that kind of design work is:

“If you can’t plan for something, then plan around it instead so that it just never happens!”

Designing fail-safe isn’t all about making sure a machine switches off safely, sometimes machines have to switch on too.

In a project a friend (mentor) worked on in the 80’s, he designed a way so that UV bug killing lights could be switched on and off based on the depth of the water in the trough, the higher the water, the more lights on, the lower, less lights needed.

Since the depth of the water in the troughs is usually lower than their maximum, these systems (literally a black box) had the potential to save million of dollars a year for a single company! Yes, they did save money (…and my friend didn’t do too bad either. Trogan, look ‘em up!)

What would you want happening if there’s a failure in this system ? You’d want all the lights on… full stop! That is the safest mode for water treatment plants to insure public health.

Back to the acoustic tech… find me a failure mode I can trust… that isn’t another acoustic generator.

I already know the answer to that one! ;)

Anybody?

tinker

FormerlyMQ

My bet would be to make sure the tube isn’t overly complex or expensive. I’m imagining something simple.

Rather than applying all the money to the tube, lets try to think how to pile all this tech onto the vehicle itself, because some posters had some good points regarding repairs – the whole loop would need to be shut down for repairs along its run rather than just removing the actual vehicle and working on it outside of the loop. It would also have to be element-proof, and easily inspected and repaired after an earthquake like event. This directly implies that the tube should be simple. Dead simple. Something that can be checked by a normal structural engineer, not someone from CERN. I’m thinking concrete sewer pipe simple. More on that in a moment.

Musk’s price tag is telling us nothing is crazy custom here. Its CHEAPER than a regular train. Lets get started.

The tube –

Simple. concrete, or steel, maybe both depending on where it is. Like concrete sewer pipes but much larger to permit a vehicle that can carry multiple people for cost efficiency.

The acceleration-

The pod/train would be accelerated using magnets hence the railgun reference. This tech is already available on roller coasters, something that has TONS of engineering behind it which makes it tried & true (& cheap!) It would help the train reach a set speed comfortably for the passengers before the actual concord part kicks in.

The pod/vehicle

The vehicle is a pod or pod train that is powered by a jet or turbofan – nothing crazy new here. This jet would be providing propulsion akin to the concord. There would be shunts that would take some of that thrust and port them out the sides of the pod/train. These jets create the air hockey cushion along the sides of the train/pod. The same principles already in practice with rotor wash from helicopters and also harrier jets. Old tech=simpler.

The deceleration

Im thinking neodymium magnetic brakes. All solid state, no electricity needed using the eddy effect. Also used on roller-coasters all over the place. If you even want to go this far into thinking about it, the vehicle could have landing gear that would basically be tires that retract into the skin of the vehicle. Just like a plane.

It is so un-romnatically simple. Ugly even. But probably the most foolproof. AND CHEAPEST! :D

Prakash Acharya

A magnetically levitated train, propelled by magnetic induction in an airtight tube filled with helium at atmospheric pressure makes the most sense to me. Helium has a density that is 7 times less than atmosphere, which allows the train to reach a velocity 2.65 times higher using the same amount of force. It would be safer than a vacuum tunnel in a way such that a breach would not cause an immediate deceleration, and there is no risk of hypobaric injury if the interface between the train and the tube is breached. A streamlined design like the concorde would still be needed to minimize the drag coeffecient.
What do you people think?

Carl Davis

Combinations of traveling opportunities and freedoms. World wide.
The “future of our country” …… should we decide to invest. I believe.
We should. Be better than what we’re getting. Eh? middle class?
Bet if oil and its politics had not wasted so much of our time these decades, we would already be 30 years minimum into advanced AMERICAN transportation.

A Concorde nose in a perforated tube? The nose cone could be articulated as to cause the air or gas pressure to be adjustable between the perforated shroud and the conduit wall. While going around bends, one would want to increase pressure to the outside of the bend. Also at slower speeds more pressure beneath the vehicle. When I say “perforated shroud” think a wingless Concorde inside of a perforated tube. Propulsion could be induction or induction powered ducted fan. As Prakash Acharya mention one could fill the system with He. Concorde/air hockey/rail gun.

lwebb82

Here’s another idea. How about we monitor all the Kepler-found exo-planets (whose technology is probably millions of years ahead of us) and see what THEY use for public transportation.

Tony

While The Hyperloop is a much faster monorail/train system it doesn’t really solve any issues that current public transportation already offers – the ability to go from place A to place B along a designated route. Our BiModal Glideway would be a better solution as it would provide the same high speed distance travel, but allow drivers the freedom to go from door to door during their commute.

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